In photographic imaging systems, such as those used for information storage and retrieval, photographic film transport and positioning arrangements may commonly employ film positioning and support structures known as gates. The purpose of the gate is to maintain a segment of photographic film relative to the focal plane of imaging optics and to permit light to be selectively directed through the film segment.
Desirably, the film gate should be such that physical contact between the film surface and a friction imparting element is prevented, in order to avoid scratching the film, which could possibly cause a loss of information contained in an image frame. To this end, improvements in film gate structures have lead to the development of fluid gates, through which a curtain, or stream, of air is generated in the area of the gate so as to provide a medium by way of which the film may be frictionlessly supported. An example of such an air pressure gate is described in U.S. Pat. No. 3,457,007 to Conroy, and consists of a differential pressure gate for a motion picture projector having a plurality of peripheral ducts which pass through the walls of the gate aperture. Air pressure is introduced through these ducts in an effort to apply a substantially uniform pressure throughout a chamber disposed on one side of a photographic film. The purpose of the differential pressure applied to a frame of motion picture film as it passes through the gate is to limit the excursion of the film in one direction and to stabilize the film. A similar pressure control arrangement for a film gate is described in German Auslegeschrift No. 1225045 and consists of a pair of parallel-positioned transparent plates between which the film is advanced. A plurality of apertures or air ducts are distributed around the periphery of the gate for applying compressed air against each side of the film, so as to maintain the motion picture film within a small spacing between the transparent plates.
Another film advance and support system which employs pressurized air flow is described in U.S. Pat. No. 3,103,850 to Khoury et al. Like the support arrangements described in the above-discussed publications, the film handling apparatus described in the Khoury et al patent contains a plurality of orifices disposed at the periphery of the support for directing air against opposite surfaces of a film strip in an effort to exert stabilizing forces on the film strip and to maintain the film strip substantially equidistant between the opposite internal surfaces of the film gate.
Now, although the film gate arrangements discussed briefly above purport to stabilize the position of a film strip within the gate, they are effectively limited to dynamic systems wherein the film is advanced either continuously or intermittently (with only a brief pause period) through the gate. It has been found by the present inventors that arrangements of the type described above, which employ simple orifice pressure inlets, cause the air passing through the air supply ducts to undergo a substantial temperature change, so that the air striking the film and creating the air bearing is cooled considerably relative to the air on the input side of the air bearing orifices. This cooled air creates a temperature gradient across the film, which may lead to curving or warping of the film. This condition is particularly noticeable for a static condition wherein the film, rather than being rapidly advanced past the film gate, is held stationary for a period of time considerably longer than even the temporary interruption of movement in a motion picture type of system.
As a result of this temperature differential created by previously proposed film gate arrangements, such systems are unsuitable for supporting films that may be required to be maintained in a static condition in the film gate and/or which require extremely accurate positioning relative to the imaging optics focal plane. This latter requirement is of paramount importance in holographic recording and reproduction optical systems wherein the tolerances desired may be on the order of several microns.